Numerous designs of fuel supply systems for multijet airplanes have been known. For example, the twin-jet Airbus A 310 has five tanks, namely, one inner wing tank and one outer wing tank on each side, as well as a middle tank in the area in which the airplane wing passes through the fuselage. The two left-hand wing tanks are used exclusively to supply the left-hand engine during normal flight operation, and the right-hand fuel tank is correspondingly used exclusively for the right-hand engine. Both engines can be supplied with fuel (kerosene) from the middle tank. Fuel transfer from tank to tank is not provided for. To make it possible to use the fuel present in the wing tank of an engine that fails for the remaining, intact engine, there is a line connection between the left-hand side and the right-hand side of the airplane with a shut-off valve that opens in the case of an emergency. This valve is called a transfer valve, a cross-feed valve or crossover valve, etc. Thus, it makes possible a direct fuel feed from the wing tanks of one side to the engine on the other side of the airplane.
The arrangement of the tanks of the four-jet Airbus A 340 is comparable to that of the A 310, but there is an additional trimming tank in the horizontal tail unit, which is or can be connected to the middle tank. There are differences inasmuch as both engines of one side are supplied basically from the inner wing tank, which again are or can be connected to the outer wing tank. However, the "crossover" principle is put into practice here as well, i.e., there is a connection between the wing tanks of one side and the engines of the other side of the airplane, which is to be opened in the case of an emergency.
In airplanes flying on cryogenic fuel, the fuel is stored in the cryogenic and liquid state in the heat-insulated tanks of the airplane. During normal flight operation, the fuel is fed to the engines as a cryogenic liquid, and the line system with its installed parts assumes approximately the temperature of the fuel.
In contrast, phases of operation during which the line system or parts thereof are still approximately at the ambient temperature, i.e., they are relatively "warm," are problematic. This happens, e.g., during the start phase of the engines, as well as during flight in fuel-filled, inactive line sections or elements, i.e., line sections or elements through which no fuel flows. Intense evaporation of the fuel, leading to great variations in pressure and flow, develops in the warm line elements at the beginning of flow. This in turn leads to an uneven, unreliable fuel flow, and in extreme cases even to a stalling of the engines.
The same effects would occur when the operation is switched over to "crossover" operation as a consequence of the failure of an engine or of a line damage. If the "crossover valve," which is at ambient temperature, and the line elements belonging to it came into contact with the liquid fuel, intense evaporation would occur here as well, so that partially gaseous, partially liquid fuel under greatly varying pressure and in greatly varying amounts would reach the running engine or the running engines.
This would lead at least to a highly erratic operation of the engine, and the intact engine or the intact engines could even fail in the worst case.
In the case of a damage to the "crossover" valve, which leads to leakage, the fuel supply of the still intact engines from their own tanks would be jeopardized as well.